Gas-liquid contactors and absorbers are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO.sub.2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such gases are known to be hazardous to the environment, and their emission into the atmosphere is closely regulated by clean air statutes. The method by which these gases are removed with a gas-liquid contactor or absorber is known as wet flue gas desulfurization (FGD).
The cleansing action produced by gas-liquid contactors and absorbers is generally derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that absorbs sulfur dioxide. Wet flue gas desulfurization processes have typically involved the use of an alkaline scrubbing liquid, such as a calcium-based slurry or a sodium-based or ammonia-based solution. As used herein, a slurry is a mixture of solids and liquids in which the content of the solids can be any desired level, including the extreme condition in which the slurry is termed a moist solid. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO.sub.3) slurries and hydrated lime (calcium hydroxide; Ca(OH).sub.2) slurries formed by action of water on lime (calcium oxide; CaO). Such slurries react with the acidic gases to form precipitates that can be collected for disposal, recycling or sale. Intimate contact between the alkaline slurry and acidic gases that are present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry and the formation of salts, such as calcium sulfite (CaSO.sub.3.1/2H.sub.2 O), gypsum (CaSO.sub.4.2H.sub.2 O), calcium chloride (CaCl.sub.2) and calcium fluoride (CaF.sub.2). When desired, forced oxidation of the slurry by aeration is employed to ensure that all of the sulfites will be reacted to form sulfates, and thereby maximize the production of gypsum.
While gas-liquid contactors and absorbers utilizing calcium-based slurries as described above generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate fertilizer. In these processes, sulfur dioxide is absorbed from flue gases with an ammonium sulfate solution or liquor, after which the sulfur dioxide is reacted with oxygen and anhydrous or aqueous ammonia injected into the solution to form additional ammonium sulfate solution or ammonium sulfate crystals ((NH.sub.4) .sub.2 SO.sub.4). Particular examples of such processes are disclosed in U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention. U.S. Pat. No. 4,690,807 discloses the use of a saturated ammonium sulfate solution, while U.S. Pat. No. 5,362,458 discloses the more conventional use of a dilute (subsaturated) solution. In addition to being required to react with sulfur dioxide to produce ammonium sulfate, ammonia also serves to increase the efficiency of sulfur dioxide removal by reducing the acidity of the ammonium sulfate solution, which becomes more acidic with the absorption of sulfur dioxide.
An ongoing demand in processes such as those taught in U.S. Pat. Nos. 4,690,807 and 5,362,458 is the ability to control ammonia slip, which is free ammonia in the scrubbed flue gases exiting the gas contactor or absorber. In addition to incurring an economic loss because of lost ammonia, free ammonia in the scrubbed flue gases reacts with uncaptured sulfur dioxide and trioxide to create an ammonium sulfate aerosol that is visible as a blue or white plume in the stack discharge, leading to secondary pollution problems. Controlling the amount of free ammonia in the desulfurization process is in part a function of the ammonia vapor pressure, which results from a combination of pH and levels of unoxidized ammonium sulfite produced by the reaction of sulfur dioxide and ammonia in the absence of sufficient oxygen. High pH values result in high ammonia vapor pressure, which promotes ammonia slip. High levels of unoxidized ammonium sulfite also promote ammonia slip.
Generally speaking, the use and addition of anhydrous or aqueous ammonia to control sulfur oxide gases have resulted in undesirable levels of ammonia slip and associated poor aerosol control. Accordingly, it would be desirable if a flue gas desulfurization process were available that involved the addition of anhydrous or aqueous ammonia while controlling ammonia slip.